1. Field of the Invention
The present invention relates to a video signal processing method and a video signal processing device each of which is applied to an image-pickup apparatus (e.g., video camera or the like) and performs a predetermined correction process to a luminance signal of a video signal generated by an image pickup operation performed by a solid state image pickup device.
2. Description of Related Art
Conventionally, in a so-called single-plate color video camera using only one sold state image pickup device (charge coupled device: CCD), color filters having different spectral characteristics are arranged on the solid state image pickup device in units of pixels, and signals depending on color components of an object image are obtained from the respective pixels, thereby generating a color video signal. For the video signal generated as described above, color components of the object image are sequentially output according to the arrangement of the color filters, and the color signals are multiplexed. Since the sold state image pickup device can know a specific pixel which a read signal is obtained from, a color signal can be easily separated and demodulated from an output video signal from the solid state image pickup device.
It is an important factor to increase the sensitivity of this single-plate color video camera. Therefore, as color filters employed in the single-plate color video camera, a complementary color filter having transmittance for white light which is higher than that of primary color filters such as R (red), G (green), and B (blue) filters are frequently used.
As the complementary color filter, a filter obtained by arranging Mg (magenta), G (green), Cy (cyan), and Ye (yellow) filters or a filter obtained by arranging W (white), G, Cy, and Ye filters is employed.
In particular, a complementary color filter obtained by arranging W, G, Cy, and Ye filters shown in FIG. 1 is frequently employed in a solid state image pickup device, which is often used in recent years, using a so-called all-pixel reading system in which signal charges of all pixels are independently read for a one-field period and output without mixing them so that high horizontal and vertical resolutions of an object can be obtained, even if the object is quickly moving.
When the complementary color filter obtained by arranging the W, G, Cy, and Ye shown in FIG. 1 is arranged in a solid state image pickup device which employs the all-pixel reading system as a reading system, a spectral difference may be often generated, as indicated by a hatched portion in FIG. 2, between a luminance signal (Ya=W+G) consisting of W and G and indicated by Ya in FIG. 2 and a luminance signal (Yb=Cy+Ye) consisting of Cy and Ye and indicated by Yb in FIG. 2. It is assumed that the luminance signal Ya is used for a first field and that the luminance signal Yb is used for a second field.
As described above, assume that a spectral difference is generated between the luminance signal Ya consisting of W and G and the luminance signal Yb consisting of Cy and Ye. In this case, thereafter, when the first and second fields are formed by using the luminance signals Ya and Yb later, and an image is displayed on a display device, flickers are generated on the screen of the display device.
Since the spectral difference as described above is caused by a characteristic difference between color filters of a complementary color filter arranged on a solid state image pickup device, it is very difficult (virtually, impossible) to correct the spectral difference by signal processing performed by a circuit arrangement. Therefore, flickers cannot be suppressed.
By the way, the basic processing procedure of a color video camera includes the process of causing a solid state image pickup device to detect an image as charges accumulated in a photoelectric conversion element and the process of causing a low-pass filter to convert a digital signal, obtained by quantizing a quantity of charge, into an analog image signal.
Although a substantial resolution is determined by the number of pixels arranged on a CCD solid state image pickup device, after a sampling process and a restoring process, a false signal caused by an essentially high spatial frequency component is inevitably generated.
FIG. 3 shows an example of a color filter array. Here, a color filter array consisting of W (white), G (green), Cy (cyan), and Ye (yellow) is used, and the four pixels are used as a base unit. In FIG. 3, reference symbol Px denotes a pixel pitch in the horizontal direction, and reference symbol Py denotes a pixel pitch in the vertical direction.
With respect to the horizontal direction, a luminance resolution corresponding a spatial frequency of 1/Px can be expected. However, for example, due to the difference between the intensities of G and Ye, the following false signal is generated when a carrier frequency is represented by fs:
(xcfx80/2)xc3x97(Wxe2x88x92G)sin(2xcfx80xc3x97fsxc3x97t)
It is an object of the present invention to provide a video signal processing method and a video signal processing device each of which can easily suppress flickers based on the characteristic difference of a complimentary color filter by signal processing performed by a circuit arrangement.
It is another object of the present invention to provide a luminance signal processing circuit for a color video camera which can considerably suppress a false signal generated by luminance signal processing for a color video camera.
To achieve the object described above, from the first aspect of the present invention, there is provided a video signal processing method of processing a luminance signal of a video signal generated by causing a solid state image pickup device to receive light transmitted through color filters, comprising the steps of: calculating an output signal from a specific pixel of the solid state image pickup device; calculating an average of output signals from all pixels of the solid state image pickup device; generating a control signal by a predetermined arithmetic operation using the output signal from the specific pixel and the average of the output signals from all the pixels of the solid state image pickup device; and performing a predetermined correction process to the luminance signal by using the control signal.
Further, from the second aspect of the present invention, there is provided a video signal processing method of processing a luminance signal of a video signal generated by causing a solid state image pickup device to receive light transmitted through color filters, comprising the steps of: calculating an output signal from a first specific pixel of a solid state image pickup device; calculating an output signal from a second specific pixel of the solid state image pickup device; generating a control signal by a predetermined arithmetic operation using the output signal from the first specific pixel and the output signal from the second specific pixel of the solid state image pickup device; and performing a predetermined correction process to a luminance signal by using the control signal.
Further, from the third aspect of the present invention, there is provided a video signal processing device for processing a luminance signal of a video signal generated by causing a solid state image pickup device to receive light transmitted through color filters, comprising: first signal operation means for calculating an output signal from a specific pixel of the solid state image pickup device; second signal operation means for calculating an average of output signals from all pixels of the solid state image pickup device; control signal generation means for generating a control signal by a predetermined arithmetic operation using the output signal from the specific pixel and the average of the output signals from all the pixels of the solid state image pickup device; and correction means for performing a predetermined correction process to the luminance signal by using the control signal.
Further, from the fourth aspect of the present invention, there is provided a video signal processing device for processing a luminance signal of a video signal generated by causing a solid state image pickup device to receive light transmitted through color filters, comprising: first signal operation means for calculating an output signal from a first specific pixel of a solid state image pickup device; second signal operation means for calculating an output signal from a second specific pixel of the solid state image pickup device; control signal generation means for generating a control signal by a predetermined arithmetic operation using the output signal from the first specific pixel and the output signal from the second specific pixel of the solid state image pickup device; and correction means for performing a predetermined correction process to a luminance signal by using the control signal.
In the video signal processing method and the video signal processing device, the predetermined arithmetic operation is performed by using the output signal from the specific pixel of the solid state image pickup device and the output signals (or output signal from another specific pixel) of all the pixels, so that the control signal having a value corresponding to a characteristic difference of color filters is generated. Therefore, when a correction process is performed to a video signal by using the control signal, the characteristic difference of the color filters can be suppressed. As a result, flickers can be suppressed from being generated.
Further, from the fifth aspect of the present invention, there is provided a luminance signal processing circuit for a color video camera, generating a luminance signal from an output signal from a solid state image pickup device having a plurality of lines obtained by repeating a pair of pixels having color filters, the luminance signal processing circuit comprising: a circuit in which a light transmittance of one of the pair of pixels and a light transmittance of the other are represented by T1 and T2, respectively, a photo detection signal from one of the pair of pixels is multiplied by (T1+T2)/(2xc3x97T1) in advance, a photo detection signal from the other of the pair of pixels is multiplied by (T1+T2)/(2xc3x97T2), and the luminance signal is generated by a low-frequency component of a signal obtained by synthesizing these photo detection signals.
Preferably, the solid state image pickup device may be a CCD solid state image pickup device. A combination of the color filters of the pair of pixels may be a combination having a small variation in color temperature. In addition, the combination of the color filters may be a combination of a filter having a transparent spectrum or an almost transparent spectrum and a green filter. Furthermore, the combination of the color filters may be a combination of a magenta filter and a green filter.
Furthermore, from the sixth aspect of the present invention, there is provided a luminance signal processing circuit for a color video camera, generating a luminance signal from an output signal from a solid state image pickup device having a plurality of lines obtained by repeating a pair of pixels having color filters, the luminance signal processing circuit comprising: a circuit in which an output signal from one of the pair of pixels and an output signal from the other are represented by T1 and T2, respectively, low-frequency components of the output signals T1 and T2 are represented by T1L and T2L, respectively, and conversions expressed by the following equations:
T1xe2x80x2=T1xc3x97(T1L+T2L)/(2xc3x97T2L)
T2xe2x80x2=T2xc3x97(T1L+T2L)/(2xc3x97T1L)
are performed to the pixels of the respective lines to generate the luminance signal.
Further, from the seventh aspect of the present invention, there is provided a luminance signal processing circuit for a color video camera, generating a luminance signal from an output signal from a solid state image pickup device having a plurality of lines obtained by repeating a pair of pixels having color filters, the luminance signal processing circuit comprising: a circuit in which an output signal from one of the pair of pixels and an output signal from the other are represented by T1 and T2, respectively, low-frequency components of the output signals T1 and T2 are represented by T1L and T2L, respectively, and conversions expressed by the following equations:
T1xe2x80x2=2xc3x97T1xc3x97T2L/(T1L+T2L)
T2xe2x80x2=2xc3x97T2xc3x97T1L/(T1L+T2L)
are performed to the pixels of the respective lines to generate the luminance signal.
Preferably, the solid state image pickup device may be a CCD solid state image pickup device. The combination of the color filters may be a combination of a filter having a transparent spectrum or an almost transparent spectrum and a green filter. In addition, the combination of the color filters may be a combination of a magenta filter and a green filter.